A large percentage of the population is affected by hearing loss or impairment. For example, the National Institute on Deafness and Other Communication Disorders estimates that in the United States, 17 percent or approximately 36 million adults have some degree of hearing loss. While there is a strong correlation between age and hearing loss, a surprising study by the American Medical Association reported that approximately 15 percent of school-aged children have a hearing loss. Additionally, it has been reported that hearing problems are on the increase in all age groups.
The primary causes of hearing loss are heredity, aging, disease, trauma, ototoxic medication and long-term exposure to loud noises. Given that our day-to-day environment has become noisier in recent times, both in terms of the noise levels experienced at work and in our leisure time, undoubtedly noise induced hearing loss is the main reason that hearing problems are on the rise. Additionally, given that noise induced hearing loss is the result of both the sound pressure level (SPL) and the length of exposure, the routine and in some cases almost constant use of headphones and earbuds is certainly a contributing factor to the noted increase, especially in younger age groups.
By their very design, headphones and earbuds place the speaker transducers in close proximity to the user's eardrums. While this configuration may be convenient and, in some cases, provide an excellent listening experience, if the volume levels are set too high, their use can easily lead to hearing loss. Unfortunately, recent studies have shown that between 25 and 50 percent of headphone/earbud users routinely listen at volume levels high enough to cause hearing loss. Further exacerbating this problem is the fact that most users will turn up the volume level in an attempt to drown out background noise (e.g., commuting noise, co-workers, etc.).
In addition to educating people on the pitfalls of excessive volume levels, a number of products have recently come to market that attempt to control the SPL delivered through a headset. Setting the maximum SPL to an acceptable level is difficult, however, since different headphones/earbuds exhibit different sensitivities, and thus deliver different sound pressure levels for the same drive level.
One approach to limiting the SPL is to place a resistor between the headphone/earbud and the audio source. The resistor reduces the current to the headphone/earbud, thereby limiting the generated SPL. Unfortunately, in order to set the SPL to a specific, desired level, the maximum drive level from the audio source as well as the sensitivity of the headphone/earbud must be known. Therefore, this approach would require selecting a specific resistor for each source/earpiece combination.
Another often-used approach for liming the SPL is to limit the maximum output volume, i.e., drive voltage, from the audio source. This feature is included in many MP3 and Apple® music players. This approach will only work, however, if the sensitivity of the headphone/earbud is known in advance of setting the maximum sound level. Otherwise, changing the headphone/earbud from a low sensitivity earpiece to a high sensitivity earpiece without changing the maximum output level will allow the desired SPL to be exceeded. Conversely, changing the headphone/earbud from a high sensitivity earpiece to a low sensitivity earpiece may yield unacceptably low volume levels.
Yet another approach to limiting the SPL is to use a limiting circuit based on a combination of resistors and diodes. As this approach is typically only used when the sensitivity of the headphone/earbud is known, the limiting circuit may be permanently coupled to the earpiece, for example by molding the limiting circuit into the earpiece cabling. In addition to only working with a specific headset, most users find this approach unsatisfactory due to the very noticeable distortion at high SPL levels that result from the nonlinear characteristics of the diode.
While a number of approaches are currently being used to limit the sound pressure levels of headphones and earbuds, and thus prevent potential hearing damage, these approaches tend to have limited applicability due to their inability to take into account variations in source drive levels and earpiece sensitivities. Additionally, these approaches often create unacceptable levels of distortion, thus further reducing the number of people willing to use them. Accordingly, what is needed is a non-distorting SPL limiter design that can be applied to a wide range of source/earpiece combinations. The present invention provides such a limiter design.